Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 100410

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
1010 PM CST Tue Jan 9 2018

.UPDATE...For 06z Aviation discussion below


.SHORT TERM...(This evening through Wednesday)
Issued at 232 PM CST Tue Jan 9 2018

Surface analysis shows a wavy front straddling the international
border from Montana to the Dakotas then weaving southeast through
MN and WI into the Great Lakes. High pressure dominates over much
of the central and eastern CONUS while multiple centers of weak
low pressure are scattered over the western CONUS. Aloft, a broad
ridge extends nearly coast-to-coast, centered along the
Mississippi River, with deep upper level lows over the eastern
GOMEX and moving onshore the SoCal coast. The upper level low over
the west coast is expected to trek east over the Rockies and to
the OK/TX panhandle area through Wednesday afternoon while the
trough as a whole becomes aligned over the Rockies, placing a deep
southwesterly flow over the Upper Mississippi River Valley region.
Within the southwesterly flow, H7-H85 frontogenesis will aid in
helping saturate the atmosphere in advance of the larger system
expected for the beginning of the longer term. However, this
saturation will manifest in the form of fog then drizzle tonight
through late tomorrow afternoon, especially as dewpoints slowly
increase from the upper 20s and lower 30s to the middle 30s over
the next 24 hours. The warmer air temperatures should contribute
to surface temperatures remaining above freezing tonight through
tomorrow so am not expecting freezing drizzle in any significant
area or duration, plus winds will be elevated enough to mitigate
any settling of precipitation that would allow it to freeze and
become an issue. In addition, QPF continues to look very low for
tonight (0.01" at most) so this adds to the thinking that freezing
drizzle is not expected. The mild air will remain in place with
the deep southerly flow plus the added warmer air aloft so will
look for highs to again hit the upper 30s to lower 40s tomorrow,
this after lows tonight only drop to around 30 degrees.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 232 PM CST Tue Jan 9 2018

Wednesday night through Thursday night...It is in this period that
the most active and significant weather of this forecast, and thus
far this season, will take place.

The upper level southern stream low will open up within the trough
overnight Wednesday into Thursday morning but remaining neutral to
positive. This will help keep some of the precipitation amounts
down and also slightly further to the east than previous runs.
Nevertheless, the scenario of the precipitation event unfolding
looks to be the same: precipitation will start generally as rain
for much of the area while a mixture of freezing rain/rain/snow
will develop in far western MN during the evening. This mixture
will then shift east overnight through Thursday morning while the
cold air surges in with the passage of the cold front late day
Wednesday through Wednesday evening. The precipitation will
steadily change over the precipitation to all snow Wednesday
night in MN then Thursday morning in WI. The heaviest QPF amounts
will be with the southern stream low as it shifts east across the
central Plains into the Mid-Mississippi River Valley region in
conjunction with the upper low. There will be a modest
precipitation surge from the weaker northern stream low, but
latest model runs produce only around 0.2" liquid precip for much
of western thru northern portions of the WFO MPX coverage area,
thus limiting the amount of snow potential since the bulk of the
precipitation up that way will come as a rain/wintry mix than
solid snow. In the south and east, the progressing upper low will
aid the more significant southern stream surface low to enhance
frontogenesis such that QPFs in far southern MN to west-central WI
will range from 0.50" to 0.75". Even if we get to a 10:1 ratio,
that would easily put those portions of the coverage area in a
5-8" snowfall band. Ratios will not be quite that high due to the
resident warm air but the problem will be that a mesoscale narrow
band of such amounts certainly cannot be ruled out within the
broader 4-6" area and this aspect will be the most tricky of this
forecast. In addition, strong gradient winds on the backside of
the front will increase to the 20-30mph range mainly in far
western and southern Minnesota during the day on Wednesday. Even
an accumulation of 2-3" within these winds with snow that will be
changing from west/heavy flakes to a more drier/fluffy brand will
produce blizzard conditions. The upper low looks to dive further
south toward the western GOMEX which will further stretch the
longwave trough over the central states Thursday night, helping to
slingshot the surface low north into the Great Lakes and winding
down the precipitation over the area. When all is said and done,
current projections look to have 1-3" for much of central and
western MN, 3-5" for far eastern into southern MN and 4-6" for
western WI. Again, this is how the scenario looks now but this
idea is still in flux so all are urged to keep apprised of the
latest statements and forecasts.

As alluded to, the cold frontal passage will bring a return to the
arctic air this area recently experienced prior to the warm-up of
these past couple days. Falling temperatures are expected
Wednesday night through Thursday night, with "high" temperatures
on Thursday in the teens out west to the mid-upper 30s (with said
high occurring at 1201am). Thursday afternoon temperatures will
range from the single digits to the mid-teens, then collapsing to
between -5F and -15F. Wind chills in western MN will drop to -20F
to -30F.

Friday through Tuesday...The arctic air will then remain in place
through the rest of the week into early next week with no reprieve
seen anytime soon. This will be due to a resumption of the upper
level pattern of a trough over eastern NOAM and a ridge over the
west. Northwest flow will keep the influx arctic air going into
the north-central state through next week. Highs will only hit the
single digits above and below zero while lows stay in the single
digits below zero to the teens below zero. Two minor clipper
systems, one Friday into Friday night and another on Sunday, may
bring some light snow to the area but nothing significant is
expected from these systems.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1010 PM CST Tue Jan 9 2018

We still expect the development of MVFR ceilings overnight, but
it looks to be delayed by a few hours (after 09z). MVFR
visibilities will also be possible as areas of fog and drizzle
develop. So far temps are staying up above freezing, so freezing
drizzle threats are decreasing. Ceilings may degrade to IFR toward
daybreak Wednesday, then improve back to MVFR through the day
Wednesday. Winds will become gusty out of the south-southeast
overnight, then lose the gustiness on Wednesday. Expect a shift
to the northwest with the frontal passage on Wednesday night, with
precipitation becoming widespread early in the next TAF period.
Still looks to start off as a wintry mix on Wednesday night.

KMSP...High confidence in MVFR cigs developing overnight, although
it looks to be a tad later than originally thought. Expect
lowering to around or below 1500 feet between 09z and 11z. Sct
IFR level still looks possible during the morning push. Temps
are still expected to be above freezing during the window for
drizzle, so leaning toward an all-liquid scenario. The Wednesday
night round of precip does look to have a moderate potential for
freezing precipitation, however.

Thurs...IFR with SN/+SN before 21z. Improving to VFR late. Wind
NNW 15G25kts.
Fri...VFR. Wind NE 5 kts.
Sat...VFR. Wind NW 5 kts.


WI...Winter Storm Watch from late Wednesday night through Thursday
     afternoon for WIZ014>016-023>028.

MN...Winter Storm Watch from late Wednesday night through Thursday
     afternoon for MNZ041>045-047>070-073>078-082>085-091>093.



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